Everyone can agree that the death toll would be huge if a nuclear conflict broke out between India and Pakistan.

The scale of devastation depends on a bomb's size and detonation

But it is impossible to predict the scale of devastation on human life, cities and the environment if either of the South Asian nuclear powers decided to use their weapons in anger.

Some analysts have come up with hypothetical death tolls of three million or 12 million - with millions more injured - should there be a nuclear exchange.

With just two examples of nuclear bomb explosions in cities - the attacks on Hiroshima and Nagasaki more than 50 years ago - the basic mechanics of what happens are known, but it is hard to transpose the models of 1945 onto other cities.

But the prospect of such a disaster even led Indian Prime Minister Atal Behari Vajpayee to compose a poem reflecting on the cost of "the ultimate weapon".

Everything is immediately vaporised amid temperatures of up to 300 million degrees Celsius.

People are also killed by burns and the massive changes in pressure which can burst a person's lungs.

Heading outwards from the epicentre, people will suffer burns, injuries from flying debris and acute exposure to ionising radiation.

Even if those injuries would not normally be life-threatening, many people would die as local infrastructure and medical services would have been destroyed and unable to cope with the sheer numbers of casualties.

Firestorms would develop and a cloud of radioactive fallout is spread with the wind.

Radioactive fallout particles enter the water supply and are inhaled and ingested, affecting communities perhaps thousands of miles from the blast.

In Hiroshima in 1945, shockwaves had destroyed everything within a four-kilometre (2.5-mile) radius 10 seconds after the bomb exploded 567 metres above the ground.

Three days later a bomb nearly twice as large was detonated 500 metres above Nagasaki and total destruction spread about 1km.

The different effects of the two bombs blown up over Japan show how a myriad of factors determine the destructiveness of a nuclear weapon.

Dr Keith Baverstock, a regional adviser on ionising radiation and public health for the World Health Organisation, said death tolls would depend on the height at which the bomb exploded, the geography of the area, the strength of buildings, population density and many other factors.

"Even the use of one weapon if it's on a city would have a massive effect - it's unthinkable," he said.

"There would be months in which people lead the most terrible life."

The impact of a bomb would spread far outside India and Pakistan, Dr Baverstock warned - not simply in terms of the passage of the radioactive cloud but in people's response to the war.

"There should be no effort spared to stop this," Dr Baverstock said.

Mr Vajpayee's poem

Sometimes at night,
Suddenly, sleep deserts me,
My eyes open, I begin to ponder
Those scientists who invented
nuclear weapons,
On hearing the gruesome human destruction,
Of Hiroshima, Nagasaki,
How did they ever sleep
at night

Those whose invention,
Created the ultimate weapon...
Do they even for a moment,
Feel what was inflicted by them,
Was monstrous?
If they do then time will not put
them in the dock,
But if they don't,
Then history will never,
Ever forgive them.